In recent years, domestic and international communication volume has sharply increased, and long-distance transmission is carried out by an optical communication system mainly. In the optical communication system, an optical amplifier in each of transponders provided in the optical transmission path at predetermined intervals amplifies signal light that has attenuated through the transmission path.
The optical amplifier in the transponder of a popular optical communication system will be described in details, referring to a drawing.
FIG. 3 is a block diagram showing an example of the optical amplifier of the transponder in the optical communication system. When the communication system is compatible with bidirectional communication in which information can be transmitted from both of the transmitting end and the receiving end, the optical amplifier amplifies the optical signal of two transmission paths, which are a transmission line A and a transmission line B. Each of the transponders in the optical communication system includes at most eight optical amplifiers inside an air-tight housing.
The optical amplifier 10 in FIG. 3 includes an erbium-doped optical fiber (EDF) 9 for each of the transmission line A and the transmission line B, and excitation light source modules 4 and 5 that oscillate excitation light. The optical amplifier 10 also includes a control circuit 7 that controls the excitation light source modules 4 and 5, an optical coupler 6 that synthesizes the excitation lights from the two modules and splits the excitation light into two beams at a predetermined ratio, and a wavelength division multiplexing (WDM) coupler 8 that couples the excitation light and the signal light and provides the coupled light to the EDF 9.
The excitation light source module 4 includes a 974 nm pumping laser diode 1 that employs InGaAs/GaAs, and a monitoring photodiode 3. The excitation light source module 5 includes a 976 nm pumping laser diode 2 that employs InGaAs/GaAs, and a monitoring photodiode 3.
The optical amplifier 10 couples and splits the excitation lights from the two excitation light source modules with the optical coupler 6, so as to provide the excitation light of the same level to the respective EDFs 9 in the transmission line A and the transmission line B. With the mentioned configuration, even though the excitation light level of one of the excitation light source modules declines, the optical signal can be amplified in the EDF 9 with the excitation light from the other excitation light source module. Thus, a redundant configuration is provided that prevents disconnection of the system. The optical amplifier 10 also includes the control circuit 7 that controls the monitoring photodiode 3 provided in the excitation light source module so as to maintain the output at a constant level. Therefore, fluctuation of the excitation light level can be suppressed, despite temporal fluctuation of ambient temperature and the pumping laser diode element.
Optical amplifiers for use in a transponder of a communication system that provide higher reliability can be found in Patent Literatures (PTL) 1 and 2. The optical amplifier according to PTL 1 prevents the life span of an excitation light source from being shortened, and controls the excitation light source so as to output the excitation light at a constant level, thus improving the reliability. PTL 2 discloses an optical amplifier having a redundant configuration including a plurality of excitation light sources, which are connected in series so as to reduce the drive current.
In addition, a technique related to the optical amplifier is disclosed in PTL 3.